The present invention relates generally to digital logic circuits and more particularly, to level shifter circuits for shifting digital signal between two different voltage levels.
Digital logic circuits are widely used in the area of electronics and computers. However, with advances in circuit technology, the various digital logic circuits that communicate with one another may operate at different power supply voltages. For example, a digital integrated circuit may include an input/output (I/O) portion operating at a first logic level and a central logic core that operates at a second, lower supply voltage, since operating at the lower voltage reduces power consumption. On the other hand, the supply voltage for the I/O section of the integrated circuit must be kept at a higher supply voltage than the logic core. This is done to ensure a higher signal to noise ratio. For example, the I/O section may operate at supply voltages ranging from 3.3V to 5V while the logic core operates at 0.5V to 1.5V. Thus, an interface is required to ensure smooth communication between different digital logic circuits operating at different voltages.
In particular, the interface must allow a shift in the voltage of a digital signal from a high voltage level to a low voltage level and vice versa. For example, if a first digital logic circuit has an operating voltage of 1.5V and a second circuit has an operating voltage of 2.5V, then for the first circuit, a high logic signal implies a signal with a voltage between 1.0V and 1.5V. On the other hand, for the second circuit, a high logic signal implies a signal with a voltage between 2.0V and 2.5V. If the two circuits are connected, then an interface is needed to convert a logic high signal from the first circuit to a logic high signal for the second circuit. This is necessary to ensure that there is no error in reading a digital signal across the two circuits. For example, if a logic high signal (about 1.0V) generated by the first circuit is supplied to the second circuit without level shifting, then the 1.0V signal will be interpreted by the second circuit as a logic low signal rather than a logic high signal and therefore, the system will not work properly.
Similarly, another interface is required to shift a logic high signal (˜2.5V) generated by the second circuit to a logic high signal (˜1.5V) to be applied to the first circuit. A shift in voltage level from the high voltage to the low voltage level also is essential to avoid damage to the first circuit.
Accordingly, there is a need for a device that allows shifting of the voltage levels of digital signals from a high voltage level to a low voltage level and vice versa.